PC: sample packet counters from ebtables
PW/RK: flush after main printf's

The output of PW and RK are important for debugging. While saving
this output to files, part of it may be lost during a reboot
if one does flush the printf's often.
PC: remove requirement on option --file

When option --file is not used, pc prints packets (pps) and
byte (Bps) rates to STDOUT.

This is a helpful feature for debugging performance.